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Amplify clock signal?
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Amanda Robin
electronics forum beginner


Joined: 24 Nov 2005
Posts: 5

PostPosted: Fri Jul 21, 2006 2:07 am    Post subject: Amplify clock signal? Reply with quote

I bought some ECS -100 i MHz clock oscillators to use as an external
clock to set a cutoff frequency in a Maxim lowpass filter (MAX281).

If I am reading the Maxim specs right, the oscillator's output voltage
is too low. The notes say that when using an external oscillator, "the
input on the COSC pin must swing close to the power rails (V+, V-)." I
will probably power it with +1 5 V.

The notes go on to say: "Although standard 74HC00 series CMOS gates do
not guarantee CMOS levels with the source and sink currents of the COSC
pin, they will in reality drive the COSC pin. CMOS gates conforming to
standard B series output drive have the appropriate voltage levels and
current to simultaneously drive several chips."

I looked up the 74HC00 series CMOS gates and found that they are NAND
gates. Will they really operate to amplify my clock signal? What am I
missing, here? I also read some previous threads in the newsgroup that
mention using a comparator for such, which I understand a little better.

If anyone would like to try to explain this to idiot child here, thanks
in advance.

Amanda
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Don Bruder
electronics forum Guru Wannabe


Joined: 03 May 2005
Posts: 190

PostPosted: Fri Jul 21, 2006 3:55 am    Post subject: Re: Amplify clock signal? Reply with quote

In article
<amanderr-9991AF.21081520072006@newsclstr02.news.prodigy.com>,
Amanda Robin <amanderr@yahoo.com> wrote:

Quote:
I bought some ECS -100 i MHz clock oscillators to use as an external
clock to set a cutoff frequency in a Maxim lowpass filter (MAX281).

If I am reading the Maxim specs right, the oscillator's output voltage
is too low. The notes say that when using an external oscillator, "the
input on the COSC pin must swing close to the power rails (V+, V-)." I
will probably power it with +1 5 V.

The notes go on to say: "Although standard 74HC00 series CMOS gates do
not guarantee CMOS levels with the source and sink currents of the COSC
pin, they will in reality drive the COSC pin. CMOS gates conforming to
standard B series output drive have the appropriate voltage levels and
current to simultaneously drive several chips."

I looked up the 74HC00 series CMOS gates and found that they are NAND
gates. Will they really operate to amplify my clock signal? What am I
missing, here? I also read some previous threads in the newsgroup that
mention using a comparator for such, which I understand a little better.

If anyone would like to try to explain this to idiot child here, thanks
in advance.

Amanda

It boils down to this:
Your oscillators don't have enough "oomph" to directly drive the pin on
the 281. So you need to feed the oscillator output into a NAND - your
oscillator into input A, input B grounded (Or whatever other combination
works - a pair of NOTs in line with each other would do just as well) to
give you the signal at the "Y" pin at a level that *DOES* have enough
"oomph" to drive the 281's pin.

In a way, the gate is acting like an amp, but that's just a side-effect
of it doing its primary function - the logic operation - It sees your
weak-but-present 1 from the osc on A, and the grounded (0) B input, says
"1 NAND 0 equals 1", and outputs a "full strength" 1 signal, since for
gates like the 7400, a weak "on" is no different than a super-strong
"on".

And to clarify something before you confuse yourself:
The "7400 series" isn't just one chip - The 7400 is indeed a NAND gate.
But there's the 7401, and the 74243, and the 7411, and... (A whole bunch
of them with numbers that start out "74", some with letters, like
74LS127 or 74HCT04) each one doing its own thing. Don't confuse one chip
of the series with the whole series! By referring you to "the 7400
series", they're telling you that chips in that series are probably
going to do the job of boosting your clock so that your signal can be
used by the 281, even though the 281 docs may not think so.

--
Don Bruder - dakidd@sonic.net - If your "From:" address isn't on my whitelist,
or the subject of the message doesn't contain the exact text "PopperAndShadow"
somewhere, any message sent to this address will go in the garbage without my
ever knowing it arrived. Sorry... <http://www.sonic.net/~dakidd> for more info
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Amanda Robin
electronics forum beginner


Joined: 24 Nov 2005
Posts: 5

PostPosted: Fri Jul 21, 2006 11:48 am    Post subject: Re: Amplify clock signal? Reply with quote

In article <44c05029$0$96219$742ec2ed@news.sonic.net>,
Don Bruder <dakidd@sonic.net> wrote:

(a really nice explanation as to why a NAND gate in the 7400 series
would effectively "amplify" a clock signal)

Thanks very much. Off to find one that will work for me.

amanda
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